例1.计算 I[a2-x2dx (ax0). 解:令x=asint,则dx=acostdt,且 当x=0时,t=0;;x=a时,t= 原武=acos'dy yy=va2-x2 S (coad a x +2sin2t)2= 4 例1. 计算 解: 令 x = asint, 则 dx = acost dt , 当x = 0时, t = 0; , . 2 x = a 时 t = ∴ 原式 = 2 a t t a (1 cos 2 )d 2 2 0 2 = + sin 2 ) 2 1 ( 2 2 t t a = + 0 2 2 0 cos t dt 2 2 2 y = a − x o x y a 且